linux-kernel - Re: [PATCH] lglock: make lg_lock_global() actually lock globally

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Date:	Thu, 26 Aug 2010 10:55:21 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jonathan Corbet <corbet@....net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Al Viro <viro@...iv.linux.org.uk>,
	Nick Piggin <npiggin@...nel.dk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lglock: make lg_lock_global() actually lock globally

Hello,

On 08/25/2010 10:00 PM, Linus Torvalds wrote:
>> lg_lock_global() currently only acquires spinlocks for online CPUs, but
>> it's meant to lock all possible CPUs.  At Nick's suggestion, change
>> for_each_online_cpu() to for_each_possible_cpu() to get the expected
>> behavior.
> 
> Can you say what this actually matters for? Don't we do stop-machine
> for CPU hotplug anyway? And if we don't, shouldn't we? Exactly because
> otherwise "for_each_online_cpu()" is always racy (and that has nothing
> to do with the lglock).

We only do stop-machine for cpu downs not ups, so code running w/
preemption disabled is guaranteed that no cpu goes down while it's
running but not the other way around.  There are two ways to achieve
synchronization against cpu up/down operations.  One is explicitly
using get/put_online_cpus() and the other is via cpu notifiers with
proper synchronization.

So, yeah, given that there's no cpu notifier implemented, the use of
for_each_online_cpu for brlock seems fishy to me.  It probably should
use for_each_possible_cpu().

Thanks.

-- 
tejun
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